The U.S. Government has certain rights in this invention pursuant to grant awarded by the Federal Government.
The present invention is directed to an optical resonator sensor for detecting the presence of specific chemical and/or biological species.
This invention relates in general to optical resonating sensors and in particular to optical resonating sensors for detecting and discriminating the presence of specific biological, chemical, etc. substances comprising a resonator specifically modified to interact with the substance by converting the interaction between the resonator and the substance into modulated light signals.
Sensors designed to take advantage of the mechanical, electrical or optical resonating properties of a material are well-known in the art. One type of conventional optical resonating sensor employs fiberoptic techniques. Various mechanical means couple the end of the output optical fiber to a mechanical or acoustic source. The mechanical or acoustic signal from the source varies the optical coupling coefficient between the two fibers so that by measuring such coefficient, the mechanical or acoustic information can be measured. For example, in U.S. Pat. No. 4,071,753 to Fulenwider et al., the ends of an input and an output optical fiber are aligned and light is transmitted from one fiber to the other, while Fuller, U.S. Pat. No. 4,419,895 discloses a sensor comprising a pair of optical fibers which are parallel but somewhat misaligned. In the above-described type of transducers, the optical sensor comprises two optical fibers optically coupled. The optical coupling coefficient between the two fibers varies with the physical parameter to be measured, so that by measuring such coefficient, the parameter can be detected and measured.
In another type of optical sensor the physical parameter to be measured modulates the vibrational motion of a transducer. Such modulation changes the intensity of light coupled between the ends of two optical fibers so that by measuring such changes the physical parameter can be detected and measured. The transducer element may be a vibrating spring, a ferroelectric or a piezoelectric element such as a quartz crystal. Such transducers are disclosed in U.S. Pat. No. 4,345,482 to Adolfsson et al.
In yet another type of optical sensor, a physical parameter moves a piezoelectrical member coupled to a fiberoptic in a direction transverse to the light transmitted in an fiberoptic so that the modulations of the movement also modulate the light transmitted by the member. In some embodiments the physical parameter causes relative motion between two flexible light transmitting members. The change in the amount of light transmitted between the two members is detected and analyzed to determine the physical parameter of interest. Such sensors are disclosed in U.S. Pat. No. 4,678,905 to Phillips.
In all the above-described applications for optical sensors, a non-optical, often mechanical, detector provides signals which alter the characteristics of light passing through a fiberoptic. These changes in the transmission of light are then processed by electronic circuitry. Such detectors are ultimately only as sensitive as the mechanical construct and further are often subject to electromagnetic interference which is undesirable.
Optical sensors have been developed to replace these traditional mechanical/electrical sensors to measure physical variables not possible with electrical sensors and to provide better performance. Other reasons for preferring optical over electrical signal sensing and transmission is the elimination of electromagnetic interference and inherent electrical isolation. One example of such an optical sensor disclosed by Anserson, et al., is a fiberoptic biosensor-combination using evanescently coupled fiber tapers. Anserson, et al., BIOSENSORS AND BIOELECTRONICS, vol. 8, pg. 249-256, (1993). However, these sensors still have a detection resolution that is limited by the perturbations induced at the surface of the fiberoptic itself.
The whispering-gallery-mode (WGM) was discovered by Lord Rayleigh over 100 years ago in the field of acoustics. The most obvious manifestation of the whispering-gallery-mode occurs in a building which has a vaulted gallery architecture. In these buildings a sound as faint as a whisper is transmitted via a whispering-gallery-mode along the vault and is readily propagated over a long distance without loss of energy.
This type of propagation has also found application in other fields including microwave and optical techniques. The property has recently been explored in dielectric microsphere resonators owing to their ultrahigh optical quality factors or Q-factors. The Q factor is a measure of the stability of light stored within a resonator, in affect the number of periods or cycles of time before the light energy decays to a critical level, which in turn effects the sensitivity of the resonator to external perturbations. For example, typical mechanical or electrical resonators such as quartz crystals have Q-factors ranging from 100 to 1,000. In comparison, the Q factors found for some silica microsphere resonators can be as high as 8 billion, indicating that the microsphere are capable of storing light energy for as many as 8 billion light cycles, resulting in a significant increase in resonator sensitivity. Gorodetsky, et al., OPTICS LETTERS, vol. 21, pg. 453, (1996). In addition, there have been significant advances in coupling these high Q-factor resonators to sources of optical power. A recent publication reported that certain optical-fiber-taper to silica-microsphere whispering-gallery-resonator systems show critical, or ideal coupling, yielding coupling efficiencies as high as 99.8%. Knight, et al., OPTICS LETTERS, vol. 22, pg. 1129 (1997); and Cai, et al., IEEE PHOTON. TECHNOL. LETT., vol. 11, No. 6, pg. 686, (1999).
These systems have several advantages over those previously utilized, including that no sophisticated optical coupling is required as input and output light is always guided and manipulated in optical fiber, the system can attain Q""s in excess of 1 million even when the microsphere is in contact with the taper, thereby eliminating in certain cases the need for sophisticated sub-micron piezo actuators, and the system is inherently robust owing to its all-fiberoptic construction.
Despite the promise shown by these optical-fiber-taper to silica-microsphere whispering-gallery-resonator systems and their acknowledged potential to open up a range of new applications, no work has been done to explore their potential as sensors. Accordingly, as a need exists for ever more sensitive optical sensor systems to detect and discriminate the presence of substances in the environment, it is important to explore these optical-fiber-taper to silica-microsphere whispering-gallery-resonator systems potential as sensors.
The present invention is directed to a system and method for utilizing an optical resonator system to sensitively detect substances of interest. This invention utilizes a surface modified resonator coupled to a light propagating coupler to provide high sensitivity substance specific detection systems. This invention is also directed to novel methods for detecting a wide range of substances using the resonating sensor of the invention.
In one embodiment, the invention is directed to a resonating sensor comprising a source of light, a detector, a coupler for receiving and conducting light from the light source and a resonator having an outer surface. The resonator is optically coupled to the coupler to allow a portion of the light passing through the coupler to enter the resonator and to be stored within the resonator for a specified period of time, and a portion of the light resonating within the resonator to exit the resonator. The outer surface of the resonator is modified such that when a substance of interest comes into contact with the modifier, it binds with the modifier, altering the complex susceptibility of the modifier such that the interaction between the modifier and the light resonating within the resonator is detectably altered. The detector is arranged such that the interaction between the modifier and the light resonating within the resonator can be monitored and recorded.
In a preferred embodiment, the resonator is evanescently coupled to the coupler and light is resonating inside the resonator in whisper-gallery modes. In this embodiment, the resonator is preferably a silica microsphere and the coupler is preferably a dual-taper fiber waveguide.
In another preferred embodiment, the modifier is a biological or chemical modifier adapted to alter the complex susceptibility of the surface of the resonator when a specific substance interacts with the modifier such that as the substance interacts with the modifier a mode loss is induced in the resonator, decreasing the transmission of light through the resonator. In this embodiment, the detector is adapted to detect the transmission spectrum of light passing through the system such that as transmission of light through the resonator decreases the signal associated with such loss is recorded.
In yet another preferred embodiment the resonating sensor system of the present invention comprises an array of couplers, each coupler linked to an independent resonators which has been modified to interact with a unique substance. In this embodiment the couplers are arrayed on a substrate and each are in optical communication with a source of light having a unique wavelength and a detector such that all resonators can be monitored simultaneously.
In still yet another embodiment, the invention is directed to a system for the detection of substances comprising a source of light in optical communication with a series of resonating sensors as described above.
In still yet another additional embodiment, the invention is directed to a method for detecting and discriminating a substance in contact with the resonating sensor. The method comprises analyzing the air in an environment using a resonating sensor as described above.